1. Field of the Invention
The present invention relates to a temperature sensor comprising a temperature sensitive element arranged in a fluid passage for detecting the temperature of the object fluid flowing in the fluid passage.
2. Description of the Related Art
A well-known temperature sensor of this type is used on an automobile (for example, Japanese Unexamined Patent Publications No. 7-201523, No. 9-126910, etc.). This type of temperature sensor is used as an exhaust gas temperature sensor and includes a temperature sensitive element (such as a thermistor element) for detecting the temperature of an exhaust gas (object fluid) flowing in an exhaust gas passage (flow passage). The sensor is generally mounted in the exhaust gas passage in the manner shown in FIG. 1A.
This temperature sensor 200 comprises a wiring member (sheath pin) 1 including a metal outer cylinder 1a and a pair of conductive core wires 1b held in the metal outer cylinder 1a through an insulating material, a temperature sensitive element (such as a thermistor element) connected to the ends of the core wires 1b at an end of the wiring member 1, and a pair of lead wires 4 connected to the other ends of the core wires 1b at the other end of the wiring member 1 for connecting an external circuit, wherein the wiring member 1 is inserted in a hole A3 formed in a wall portion A2 defining the exhaust gas passage A1 thereby to set the temperature sensitive element 2 in position in the exhaust gas passage A1.
In this configuration, the connecting portions (lead wire coupling portions) B1 between the core wires 1b and the lead wires 4 are protected by being covered with a cylindrical protective member (sleeve) J6. This protective member J6 is coupled around the whole periphery of the weld zone (coupling portion) D1 along the outer peripheral surface of the outer cylinder 1a. Also, a seal member J5 for preventing the leakage of the exhaust gas from the hole A3 is coupled around the whole outer peripheral surface of the protective member J6 in the weld zone (coupling portion) D2.
This seal member J5, as shown in FIG. 1A, includes a tapered seat surface J5a corresponding to the tapered surface of the hole A3. By bringing the seat surface J5a in close contact with the tapered surface of the hole A3 by a screw A5, the exhaust gas is prevented from leaking from the hole A3.
Also, a metal cap 3 made of a heat-resistant metal is coupled around the whole outer peripheral surface of the outer cylinder 1a at an end of the wiring member 1. The temperature sensitive element 2 is housed in the metal cap 3 and thus is protected from the exhaust gas. The output of the temperature sensitive element 2 is retrieved by an external circuit, not shown, by way of the lead wires 4 from the core wires 1b of the wiring member 1 thereby to detect the temperature of the exhaust gas.
In the case where the temperature sensitive element 2 is disposed in the exhaust gas passage A having this constitution, however, the weld zones D1 and D2 are exposed to the exhaust gas in the exhaust gas passage A1, and therefore the exhaust gas is required to be sealed from the outside of the passage at three points including the seat surface J5a of the seal member J5 and the weld zones D1 and D2. Especially, since there are two coupling portions where maintenance of air-tightness is required, the resulting constitution is extremely unreliable from the viewpoint of the sealing of the exhaust gas.
Also, from the viewpoint of the improvement of the sensor response, the component members of the temperature sensitive portion of the sensor, i.e. the temperature sensitive element 2, the metal cap 3 and the wiring member 1 are preferably reduced in diameter. In that case, the protective member J6 coupled to the outer cylinder 1a of the wiring member 1 is also required to be reduced in diameter. The portion of the protective member J6 for protecting the lead wire coupling portions B1, on the other hand, is required to secure a sufficient space for welding or arranging a connector for connecting the core wires 1b and the lead wires 4. Therefore, the diameter of the portion of the protective member J6 for protecting the lead wire coupling portions B1 constitutes a controlling factor for determining the diameter of the protective member J6.
As is shown in FIG. 1A, however, the protective member J6 is straight in shape. The diameter of the protective member J6 cannot be reduced more than necessary, therefore, for securing the diameter of the protective portion of the lead wire coupling portions B1. Also, the diameter of the wiring member 1, which is coupled to the inner peripheral surface of the protective member J6 on the outer peripheral surface of the outer cylinder 1a thereof, is determined by the diameter of the protective member J6 and therefore cannot be easily reduced.
For improving the response of the temperature sensor, a deformed protective member (stepped sleeve) J6 shown in FIG. 1B may be employed in which the portion for protecting the lead wire coupling portions B1 and the portion D1 for coupling with the outer cylinder 1a of the wiring member 1 have different diameters. This constitution requires, however, not only the cutting of the pipe but the drawing on machining of the protective member, thereby leading to a considerably increased machining cost for an increased production cost.
The problems of sealing and the protective member described above are not confined to the temperature sensor for detecting the exhaust gas temperature. Specifically, the problems are shared by temperature sensors comprising a wiring member with one end connected to a temperature sensitive element and the other end connected to lead wires and a cylindrical protective member for covering the connecting portion of the lead wires, wherein the wiring member is arranged in the fluid passage by being inserted in a hole formed in the wall portion of a fluid passage.